Implantable cardioverter defibrillator (ICD) systems deliver a high voltage electrical countershock to the heart in an attempt to correct or convert a detected cardiac dysrhythmia or fibrillation. Due to the limitations on size and power imposed by the fact that these systems must be self-contained implantable devices, all existing ICD systems generate an electrical countershock by charging a capacitor system to a high voltage from a low voltage battery and oscillator circuit. The battery is then switched out of the circuit and the electrical charge stored in the capacitor system is delivered as a truncated capacitive discharge through two or more implanted electrodes.
To date, there have been two basic kinds of discharge waveforms which have been used with ICD systems: monophasic waveforms and biphasic waveforms; both of which are delivered as a truncated capacitive discharge. Monophasic waveforms are comprised of a single monotonically decaying electrical pulse that is typically truncated before the capacitor system is completely discharged. Biphasic waveforms, on the other hand, are comprised of a decaying electrical pulse that has a pair of decaying electrical phases that are of opposite polarity. To generate a biphasic pulse an H-bridge switch circuit connected to the electrodes is used to switch the polarity of the two phases. In generating the biphasic pulse, a first phase is discharged from the capacitor system, much in the same manner as a monophasic pulse. At the point in time that the first pulse is truncated, the H-bridge switch circuit immediately reverses the discharge polarity of the capacitor system as seen by the electrodes to produce the second phase of the biphasic waveform that is of the opposite polarity. A typical example of the use of an H-bridge circuit to generate a biphasic waveform in an implantable defibrillator system is shown in U.S. Pat. No. 4,998,531.
Over the last twenty five years, it has been demonstrated that appropriately truncated biphasic waveforms can achieve defibrillation with significantly lower currents, voltages and energies than monophasic waveforms of similar durations. Kroll, M W et al., "Decline in Defibrillation Thresholds", PACE 1993; 16#1:213-217; Bardy, G H et al., "A Prospective Randomized Evaluation of Biphasic vs. Monophasic Waveform Pulses on Defibrillation Efficiency in Humans", J American College of Cardiology, 1989; 14:728-733; and Wyse, D G et al., "Comparison of Biphasic and Monophasic Shocks for Defibrillation using a Non-Thoracotomy System", American J Cardiology 1993; 71:197-202. These findings are of particular importance for implantable devices because of the direct relationship between the amount of energy required for defibrillation and the overall size of the implantable device, i.e., the lower the energy required for defibrillation, the smaller the device.
Numerous theories have been advanced to explain the improved efficiency of the biphasic waveform over the more conventional monophasic waveform. Although some of these theories may partly explain, or may act cooperatively to explain, the effect a biphasic waveform has on the heart, there is currently no single accepted theory which fully explains the advantages of the biphasic waveform over the monophasic waveform. As a result, there is little or no agreement on what factors might further improve the efficiency and operation of the biphasic waveform.
In U.S. Pat. No. 5,199,429, a system for delivering a novel biphasic waveform is described in which two capacitor systems are used to store the electrical charge for the electrical countershock. To generate this biphasic waveform, the capacitor systems are configured in parallel for delivering a first phase of the biphasic waveform and in series for delivering a second phase of the biphasic waveform.
In U.S. Pat. No. 5,871,505, a new model is presented for understanding why the biphasic waveform is more effective than a monophasic waveform. Applying this new model, another system for delivering a novel biphasic waveform is described in which two capacitor systems are used to store the electrical charge for the electrical countershock. A first phase of the biphasic waveform for this system is delivered from a first electrical charge stored in a first capacitor system, and a second phase of the biphasic waveform is delivered from a second electrical charge stored in a second capacitor system. To maximize the effectiveness of the biphasic waveform according to the new model, this system teaches that the second electrical charge should be of less energy and should be stored separate and distinct from the first electrical charge.
In U.S. Pat. No. 5,833,712, a method and apparatus for generating biphasic waveforms uses an implantable cardioverter defibrillator having two capacitor systems and a switching network. A first phase of the biphasic waveform is produced by configuring the two capacitor systems to selectively discharge first in a parallel combination, and then in a series combination. The second phase of the biphasic waveform is produced by reconfiguring the two capacitor systems in a parallel combination. By reverting to a parallel configuration for the second phase of the biphasic waveform, the output characteristics of the second phase of a biphasic waveform more closely matched the model for understanding the effectiveness of the biphasic waveform.
Although these systems for generating biphasic waveforms have significantly improved the overall effectiveness of the biphasic countershock as delivered by an ICD, it would be desirable to further improve the effectiveness of such biphasic waveforms.